Onslaught from Radiant Crusade could be more consistent

I've been playing guardian in sanctum using summon phantasms with black cane, and I'm noticing how finicky the Radiant Crusade onslaught effect is.

The phantasms are quick to spread out, so I found I couldn't rely on them to keep onslaught active when I want to move. I've tried adding a golem and heralds of purity to my setup, but even with them I have cases where I'm moving and my minions are following, but they're just outside the radius to keep it active.

My character only has 27% movement speed without onslaught, so if as slow a character as mine is outrunning my minions, I really think that it would be good if radiant crusade provided its effect for a duration, to cover up some of the downtime.

I'm sure the wording and duration might be different but something like "You and nearby allies gain Onslaught for 4 seconds when you have at least five nearby allies"